Systems and methods for minimizing scattered light in multi-SLM maskless lithography

ABSTRACT

The present invention is directed to a lithography system in which scattered light from a pattern generator, having one or more pattern generating devices, is blocked from an object, such as a wafer or display. The system includes a pattern generator with multiple pattern generating devices, a projection system for directing light from the pattern generating device, and an aperture located at or near an object window. The aperture has a profile that matches the configuration of the pattern generating devices. A method for blocking scattered light in a lithography system using multiple pattern generating devices is also provided.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to lithography, and more particularly, tomaskless lithography involving one or more pattern generating devices.

2. Background of Invention

Lithography systems are used to print features in a variety ofmanufacturing applications. Photolithography systems use a mask orreticle to expose features onto an object. In semiconductormanufacturing, for example, a reticle is exposed by an exposure beam. Anoptical system then projects a reduced image of the reticle onto asilicon wafer. In this way, circuit features can be printed on asemiconductor substrate.

Maskless lithography systems have been developed that do not require useof a mask or reticle. Current maskless lithography systems project apattern to be printed onto a moving object. For example, a pattern ofcircuit features can be projected onto a moving wafer or flat paneldisplay. In one example, a silicon wafer can be coated with aphotoresist. The pattern is projected on the wafer using one or morepattern generating devices, such as one or more spatial light modulators(SLM). Types of SLMs can include, for example, digital micromirrordevices (DMD), transmissive liquid crystal light valves (LCLV), andgrating light valves (GLV).

In maskless lithography involving multiple pattern generating devices,such as multi-SLM maskless lithography, multiple SLMs are typically usedin a flat plan to generate a pattern onto a work surface, such as awafer or flat panel display. Relatively large gaps can exist betweenSLMs in a multi-SLM maskless lithography system. These gaps lead to alarge amount of scattered light reaching a wafer in areas between theactive portions of the SLMs, which should remain dark. Improper dosecontrol and loss of contrast in the wafer imaging, for example, canresult that leads to degraded imaging and reduced circuit performancefor the device being produced.

What are needed are systems and methods for minimizing scattered lightin multi-pattern generating device maskless lithography systems.

SUMMARY OF THE INVENTION

The present invention is directed to a lithography system in whichscattered light from pattern generating devices, such as an SLMs, isblocked from an object, such as a wafer or display. The system includesa pattern generator with one or more pattern generating devices, aprojection system for directing light from the pattern generatingdevice, and an aperture located at or near an object window. Theaperture has a profile that matches the configuration of the patterngenerating devices. A method for blocking scattered light in alithography system using multiple pattern generating devices is alsoprovided.

Further embodiments, features, and advantages of the invention, as wellas the structure and operation of the various embodiments of theinvention are described in detail below with reference to accompanyingdrawings.

BRIEF DESCRIPTION OF THE FIGURES

The invention is described with reference to the accompanying drawings.In the drawings, like reference numbers indicate identical orfunctionally similar elements. The drawing in which an element firstappears is indicated by the left-most digit in the correspondingreference number.

FIG. 1 is a diagram of a lithographic projection apparatus, according toan embodiment of the invention.

FIG. 2 is a diagram of a lithographic projection apparatus highlightinga wafer window, according to an embodiment of the invention.

FIG. 3A is a diagram of a symmetric spatial light modulator (SLM) arraylayout, according to an embodiment of the invention.

FIG. 3B is a diagram of an aperture profile matching a symmetric SLMarray layout, according to an embodiment of the invention.

FIG. 3C is a diagram of an overlay of an aperture profile and asymmetric SLM array layout, according to an embodiment of the invention.

FIG. 4A is a diagram of an aperture profile matching a symmetric SLMarray layout, according to an embodiment of the invention.

FIG. 4B is a diagram of an overlay of an aperture profile and asymmetric SLM array layout, according to an embodiment of the invention.

FIG. 5 is a flowchart of a method for reducing scattered light in amulti-pattern generating device maskless lithography system, accordingto an embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

While the present invention is described herein with reference toillustrative embodiments for particular applications, it should beunderstood that the invention is not limited thereto. Those skilled inthe art with access to the teachings provided herein will recognizeadditional modifications, applications, and embodiments within the scopethereof and additional fields in which the invention would be ofsignificant utility.

The term “pattern generator” as here employed should be broadlyinterpreted as referring to any device that can be used to endow anincoming radiation beam with a patterned cross-section, so that adesired pattern can be created in a target portion of the substrate. Theterms “light valve” and “Spatial Light Modulator” (SLM) can also be usedin this context. Examples of such patterning devices are discussedbelow.

A programmable mirror array may comprise a matrix-addressable surfacehaving a viscoelastic control layer and a reflective surface. The basicprinciple behind such an apparatus is that, for example, addressed areasof the reflective surface reflect incident light as diffracted light,whereas unaddressed areas reflect incident light as undiffracted light.Using an appropriate spatial filter, the undiffracted light can befiltered out of the reflected beam, leaving only the diffracted light toreach the substrate. In this manner, the beam becomes patternedaccording to the addressing pattern of the matrix-addressable surface.

It will be appreciated that, as an alternative, the filter may filterout the diffracted light, leaving the undiffracted light to reach thesubstrate. An array of diffractive optical micro electrical mechanicalsystem (MEMS) devices can also be used in a corresponding manner. Eachdiffractive optical MEMS device can include a plurality of reflectiveribbons that can be deformed relative to one another to form a gratingthat reflects incident light as diffracted light.

A further alternative embodiment can include a programmable mirror arrayemploying a matrix arrangement of tiny mirrors, each of which can beindividually tilted about an axis by applying a suitable localizedelectric field, or by employing piezoelectric actuation means. Onceagain, the mirrors are matrix-addressable, such that addressed mirrorswill reflect an incoming radiation beam in a different direction tounaddressed mirrors; in this manner, the reflected beam is patternedaccording to the addressing pattern of the matrix-addressable mirrors.The required matrix addressing can be performed using suitableelectronic means.

In both of the situations described here above, the pattern generatorcan comprise one or more programmable mirror arrays. More information onmirror arrays as here referred to can be gleaned, for example, from U.S.Pat. Nos. 5,296,891 and 5,523,193, and PCT patent applications WO98/38597 and WO 98/33096, which are incorporated herein by reference intheir entireties.

A programmable LCD array can also be used. An example of such aconstruction is given in U.S. Pat. No. 5,229,872, which is incorporatedherein by reference in its entirety.

It should be appreciated that where pre-biasing of features, opticalproximity correction features, phase variation techniques and multipleexposure techniques are used, for example, the pattern “displayed” onthe pattern generator may differ substantially from the patterneventually transferred to a layer of or on the substrate. Similarly, thepattern eventually generated on the substrate may not correspond to thepattern formed at any one instant on the pattern generator. This may bethe case in an arrangement in which the eventual pattern formed on eachpart of the substrate is built up over a given period of time or a givennumber of exposures during which the pattern on the pattern generatorand/or the relative position of the substrate changes.

Although specific reference may be made in this text to the use oflithographic apparatus in the manufacture of ICs, it should beunderstood that the lithographic apparatus and projection systemsdescribed herein may have other applications, such as, for example, themanufacture of DNA chips, MEMS, MOEMS, integrated optical systems,guidance and detection patterns for magnetic domain memories, flat paneldisplays, thin film magnetic heads, etc. The skilled artisan willappreciate that, in the context of such alternative applications, anyuse of the terms “wafer” or “die” herein may be considered as synonymouswith the more general terms “substrate” or “target portion”,respectively. The substrate referred to herein may be processed, beforeor after exposure, in for example a track (a tool that typically appliesa layer of resist to a substrate and develops the exposed resist) or ametrology or inspection tool. Where applicable, the disclosure hereinmay be applied to such and other substrate processing tools. Further,the substrate may be processed more than once, for example in order tocreate a multi-layer IC, so that the term substrate used herein may alsorefer to a substrate that already contains multiple processed layers.

The terms “radiation” and “beam” and “light ray” used herein encompassall types of electromagnetic radiation, including ultraviolet (UV)radiation (e.g. having a wavelength of 365, 248, 193, 157 or 126 nm) andextreme ultra-violet (EUV) radiation (e.g. having a wavelength in therange of 5-20 nm), as well as particle beams, such as ion beams orelectron beams.

The illumination system may also encompass various types of opticalcomponents, including refractive, reflective, diffractive, andcatadioptric optical components for directing, shaping, or controllingthe projection beam of radiation.

The lithographic apparatus may be of a type having two (e.g., dualstage) or more substrate tables (and/or two or more mask tables). Insuch “multiple stage” machines the additional tables may be used inparallel, or preparatory steps may be carried out on one or more tableswhile one or more other tables are being used for exposure.

The lithographic apparatus may also be of a type wherein the substrateis immersed in a liquid having a relatively high refractive index (e.g.,water), so as to fill a space between the final element of theprojection system and the substrate. Immersion liquids may also beapplied to other spaces in the lithographic apparatus, for example,between the SLM and the first element of the projection system.Immersion techniques are well known in the art for increasing thenumerical aperture of projection systems.

FIG. 1 is a diagram of lithographic projection apparatus 100, accordingto an embodiment of the invention. Apparatus 100 includes at least aradiation system 102, pattern generator 104, an object table 106 (e.g.,a substrate table), and an projection system 108.

Radiation system 102 can be used for supplying a projection beam 110 ofradiation (e.g., UV radiation), which in this particular case alsocomprises a radiation source 112.

A pattern generator 104 (e.g., spatial light modulator) can be used forapplying a pattern to projection beam 110. In general, the position ofpattern generator 104 can be fixed relative to projection system 108.However, in an alternative arrangement, pattern generator 104 may beconnected to a positioning device (not shown) for accurately positioningit with respect to projection system 108. As here depicted, patterngenerator 104 is of a reflective type (e.g., has a reflective array ofindividually controllable elements).

Object table 106 can be provided with a substrate holder (notspecifically shown) for holding a substrate 114 (e.g., a resist coatedsilicon wafer or glass substrate) and object table 106 can be connectedto a positioning device (not shown) for accurately positioning substrate114 with respect to projection system 108.

Projection system 108 (e.g., a quartz and/or CaF2 lens system or acatadioptric system comprising lens elements made from such materials,or a mirror system) can be used for projecting the patterned beamreceived from a beam splitter 118 onto a target portion 120 (e.g., oneor more dies) of substrate 114. Projection system 108 may project animage of pattern generator 104 onto substrate 114. Alternatively,projection system 108 may project images of secondary sources for whichthe elements of pattern generator 104 act as shutters. Projection system108 may also comprise a micro lens array (MLA) to form the secondarysources and to project microspots onto substrate 114.

Source 112 (e.g., an excimer laser) can produce a beam of radiation 122.Beam 122 is fed into an illumination system (illuminator) 124, eitherdirectly or after having traversed conditioning device 126, such as abeam expander 126, for example. Illuminator 124 may comprise anadjusting device 128 for setting the outer and/or inner radial extent(commonly referred to as σ-outer and σ-inner, respectively) of theintensity distribution in beam 122. In addition, illuminator 124 willgenerally include various other components, such as an integrator 130and a condenser 132. In this way, projection beam 110 impinging onpattern generator 104 has a desired uniformity and intensitydistribution in its cross section.

It should be noted, with regard to FIG. 1, that source 112 may be withinthe housing of lithographic projection apparatus 100 (as is often thecase when source 112 is a mercury lamp, for example). In alternativeembodiments, source 112 may also be remote from lithographic projectionapparatus 100. In this case, radiation beam 122 would be directed intoapparatus 100 (e.g., with the aid of suitable directing mirrors). Thislatter scenario is often the case when source 112 is an excimer laser.It is to be appreciated that both of these scenarios are contemplatedwithin the scope of the present invention.

Beam 110 subsequently intercepts pattern generator 104 after beingdirected using beam splitter 118. Having been reflected by patterngenerator 104, beam 110 passes through projection system 108, whichfocuses beam 110 onto a target portion 120 of the substrate 114.

With the aid of positioning device (and optionally interferometricmeasuring device 134 on a base plate 136 that receives interferometricbeams 138 via beam splitter 140), substrate table 106 can be movedaccurately, so as to position different target portions 120 in the pathof beam 110. Where used, the positioning device for the patterngenerator 104 can be used to accurately correct the position of patterngenerator 104 with respect to the path of beam 110, e.g., during a scan.In general, movement of object table 106 is realized with the aid of along-stroke module (course positioning) and a short-stroke module (finepositioning), which are not explicitly depicted in FIG. 1. A similarsystem may also be used to position pattern generator 104. It will beappreciated that projection beam 110 may alternatively/additionally bemoveable, while object table 106 and/or pattern generator 104 may have afixed position to provide the required relative movement.

In an alternative configuration of the embodiment, substrate table 106may be fixed, with substrate 114 being moveable over substrate table106. Where this is done, substrate table 106 is provided with amultitude of openings on a flat uppermost surface, gas being fed throughthe openings to provide a gas cushion which is capable of supportingsubstrate 114. This is conventionally referred to as an air bearingarrangement. Substrate 114 is moved over substrate table 106 using oneor more actuators (not shown), which are capable of accuratelypositioning substrate 114 with respect to the path of beam 110.Alternatively, substrate 114 may be moved over substrate table 106 byselectively starting and stopping the passage of gas through theopenings.

Although lithography apparatus 100 according to the invention is hereindescribed as being for exposing a resist on a substrate, it will beappreciated that the invention is not limited to this use and apparatus100 may be used to project a patterned projection beam 110 for use inresistless lithography.

The depicted apparatus 100 can be used in four preferred modes:

1. Step mode: the entire pattern from pattern generator 104 is projectedin one go (i.e., a single “flash”) onto a target portion 120. Substratetable 106 is then moved in the x and/or y directions to a differentposition for a different target portion 120 to be irradiated bypatterned projection beam 110.

2. Scan mode: essentially the same as step mode, except that a giventarget portion 120 is not exposed in a single “flash.” Instead, patterngenerator 104 is movable in a given direction (the so-called “scandirection”, e.g., the y direction) with a speed v, so that patternedprojection beam 110 is caused to scan over pattern generator 104.Concurrently, substrate table 106 is simultaneously moved in the same oropposite direction at a speed V=Mv, in which M is the magnification ofprojection system 108. In this manner, a relatively large target portion120 can be exposed, without having to compromise on resolution.

3. Pulse mode: the array of individually controllable elements 104 iskept essentially stationary and the entire pattern is projected onto atarget portion 120 of substrate 114 using pulsed radiation system 102.Substrate table 106 is moved with an essentially constant speed suchthat patterned projection beam 110 is caused to scan a line acrosssubstrate 106. The pattern on pattern generator 104 is updated asrequired between pulses of radiation system 102 and the pulses are timedsuch that successive target portions 120 are exposed at the requiredlocations on substrate 114. Consequently, patterned projection beam 110can scan across substrate 114 to expose the complete pattern for a stripof substrate 114. The process is repeated until complete substrate 114has been exposed line by line.

4. Continuous scan mode: essentially the same as pulse mode except thata substantially constant radiation system 102 is used and the pattern onpattern generator 104 is updated as patterned projection beam 110 scansacross substrate 114 and exposes it.

5. Pixel Grid Imaging Mode: the pattern formed on substrate 114 isrealized by subsequent exposure of spots formed by a spot generator thatare directed onto array 104. The exposed spots have substantially thesame shape. On substrate 114 the spots are printed in substantially agrid. In one example, the spot size is larger than a pitch of a printedpixel grid, but much smaller than the exposure spot grid. By varyingintensity of the spots printed, a pattern is realized. In between theexposure flashes the intensity distribution over the spots is varied.

Combinations and/or variations on the above described modes of use orentirely different modes of use may also be employed.

FIG. 2 is a diagram of lithographic projection apparatus 100 thathighlights an object window, according to an embodiment of theinvention. FIG. 2 provides a simplified version of the diagram oflithographic projection apparatus 100 that was discussed with respect toFIG. 1. FIG. 2 highlights that lithographic projection apparatus 100includes object window 210 between projection system 108 and substrate114. (e.g., a resist coated silicon wafer or glass substrate).

In this example, pattern generator 104 includes multiple patterngenerating devices that are aligned in columns. In other embodiments,pattern generator 104 can include a single pattern generating device.Multiple pattern generator layouts have a relatively small area per shotto be illuminated, compared to the size of the shot. The area isgenerally on the order of 10-20%, depending upon the packaging size andconfiguration of the pattern generator. Even when a custom fielddefining element and/or an aperture plate at the pattern generator isused to prevent illumination of the non-active areas, some light mayscatter in the projection optics and reach these dark areas at the waferplane.

The invention places an aperture at or near object window 210 with aprofile that matches the configuration of a multi-pattern generatingdevices layout. (e.g., a multi-SLM array layout as is depicted in FIG.3A below). The aperture profile accounts for the magnification andnumerical aperture (NA) of the projection optics. Example apertures areillustrated in FIGS. 3B and 4A below.

The aperture can be a standalone aperture plate located on or nearobject window 210. In an alternative embodiment the aperture can belithographically printed and etched directly onto object window 210.This embodiment addresses the situation that the feature sizes of theaperture near object window 210 will be relatively small. The aperturecan also be etched onto a window near an image plan or onto a lenswithin a projection optics system.

The placement of an aperture near object window 210 can reduce scatteredlight from a pattern generating device, such as pattern generator 104.Additionally, improvements can be achieved by maximizing the spacebetween the column pairs of pattern generating devices to create afairly large aperture between the column pairs where scattered lightwould be most detrimental. Additionally, this approach enhances themanufacturability of the aperture by keeping the SLM array shaped asseparate tall, thin columns.

In this case, the spacing ratios between SLMs in the column pairs wouldstill need to be properly controlled to ensure appropriate stitching, aswould be known by individuals skilled in the relevant arts, based on theteachings herein. Furthermore, when SLMs are used as the patterngenerating devices, the spacing should not exceed the maximummanufacturable lens diameter at the SLM plane.

FIG. 3A is a diagram of symmetric SLM array layout 300, according to anembodiment of the invention. SLM array layout 300 is an embodiment ofpattern generator 104. Many layouts of pattern generating devices can beused for pattern generator 104, as will be known by individuals skilledin the relevant arts. The present example is provided for the purposesof illustration, and not intended to limit the scope of the invention.Rather, the invention covers the use of all types of multi-patterngenerating device layouts.

SLM array layout 300 includes four columns of SLMs: SLM column 305, SLMcolumn 310, SLM column 315 and SLM column 320. Each SLM column includesthree SLMs. SLMs 322, 326, 330 and 334 represent the first SLM in eachcolumn respectively. SLM light reflecting portions 324, 328, 332 and 336represent the portion of the SLM that reflects light. The non-lightreflecting portions of the SLM include inactive areas, such as circuits,actuators, etc that can cause scattered or stray light to reach theobject.

FIG. 3B is a diagram of aperture profile 340 matching the layout of theSLMs in SLM array layout 300, according to an embodiment of theinvention. Aperture profile 340 would be located at or near objectwindow 210, or etched directly onto wafer window 210. Aperture profile340 includes four columns of openings that have been etched outcorresponding to the SLMs in SLM columns 305, 310, 315 and 320. Theopenings 342, 344, 346 and 348 represent the top opening in each of thefour columns of openings.

FIG. 3C is a diagram on an overlay of aperture profile 340 and SLM arraylayout 300, according to an embodiment of the invention. As seen in FIG.3C, opening 342 corresponds to SLM light reflecting portion 324. Opening344 corresponds to SLM light reflecting portion 328. Opening 346corresponds to SLM light reflecting portion 322. Opening 348 correspondsto light reflecting portion 336. Aperture profile 340 will have anopening corresponding to each of the SLMs in SLM array layout 300, suchthat the opening reduce scattered light that impinges on the wafer orother work surface.

FIG. 4A is a diagram of another example of an aperture profile, apertureprofile 410, that matches SLM array layout 300, according to anembodiment of the invention. Aperture profile 410 includes rectangularopenings, such as openings 412, 414, 416, and 418, that are etched out,or removed in another way, to block scattered light. The other areas ofaperture profile 410 will block scattered light.

FIG. 4B. illustrates an overlay of aperture profile 410 onto SLM arraylayout 300, showing how aperture profile 410 can be used to blockscattered light. Each of the rectangular openings, such as openings 412,414, 416 and 418 correspond to an SLM light reflecting portions. Forexample, opening 412 corresponds to SLM light reflecting portion 324.Many types of aperture profiles can be designed, depending on theparticular application, layout of SLMs, and other design considerations.

FIG. 5 is a flowchart of method 500 for reducing scattered light in amulti-pattern generating device maskless lithography system, accordingto an embodiment of the invention. Method 500 begins in step 500. Instep 500 a radiation beam is generated. For example, source 112 cangenerate a radiation beam. In step 520 the radiation beam is patterned.For example, pattern generator 104 can generate a desired pattern. Instep 530 the patterned radiation beam is projected toward a substrate. Asubstrate can include, for example, a wafer, a display device, a camera,a projection system display device or a projection television systemdisplay. For example, projection system 108 can direct a patternedradiation beam toward substrate 114. In step 540, scattered light fromthe radiation beam is blocked from reaching a substrate. For example,aperture 300 can be used to block scattered light, while allowingpatterned radiation to pass and ultimately impinge upon substrate 114.In step 550 method 500 ends.

CONCLUSION

Exemplary embodiments of the present invention have been presented. Theinvention is not limited to these examples. These examples are presentedherein for purposes of illustration, and not limitation. Alternatives(including equivalents, extensions, variations, deviations, etc., ofthose described herein) will be apparent to persons skilled in therelevant art(s) based on the teachings contained herein. Suchalternatives fall within the scope and spirit of the invention.

1. A system, comprising one or more pattern generating devices having apattern generating configuration; a projection optics system fordirecting light from said one or more pattern generating devices to asubstrate or work surface; and an aperture within said projection opticssystem having a profile that matches the pattern generatingconfiguration.
 2. The system of claim 1, wherein said one or morepattern generating devices are spatial light modulators.
 3. The systemof claim 2, wherein said spatial light modulators are digitalmicromirror devices.
 4. The system of claim 2, wherein said spatiallight modulators are transmissive liquid crystal light valves.
 5. Thesystem of claim 2, wherein said spatial light modulators are gratinglight values.
 6. The system of claim 1, wherein said aperture is anaperture plate.
 7. The system of claim 1, wherein said aperture isetched onto a window near an object plane of the projection opticssystem.
 8. The system of claim 1, wherein said aperture is etched onto awindow near an image plane of the projection optics system.
 9. Thesystem of claim 1, wherein said aperture is etched onto a lens withinthe projection optics system.
 10. A projection optics system for usewith one or more pattern generating devices having a pattern generatingconfiguration, comprising one or more optical elements for directinglight; an aperture having a profile with openings that correspond tolocations of the one or more pattern generating devices.
 11. Theprojection optic system of claim 10, wherein said aperture is etchedonto a window near an object plane of the projection optics system. 12.The projection optic system of claim 10, wherein said aperture is etchedonto a window near an image plane of the projection optics system. 13.The projection optic system of claim 10, wherein said aperture is etchedonto a lens within the projection optics system.
 14. A method,comprising: (a) generating a beam of radiation; (b) patterning portionsof the beam of radiation; (c) projecting the patterned beam of radiationtowards a substrate; and (d) blocking scattered light from the beam ofradiation from the object.
 15. The method of claim 14, furthercomprising providing a display device as the substrate.
 16. The methodof claim 14, further comprising providing a wafer as the substrate. 17.The method of claim 14, further comprising providing a camera as thesubstrate.